Variable thickness pads on a substrate surface

ABSTRACT

An electronic structure, and associated method of fabrication, that includes a substrate having attached circuit elements and conductive bonding pads of varying thickness. Pad categories relating to pad thickness include thick pads (17 to 50 microns), medium pads (10-17 microns), and thin pads (3 to 10 microns). A thick pad is used for coupling a ball grid array (BGA) to a substrate with attachment of the BGA to a circuit card. A medium pad is useful in flip-chip bonding of a chip to a substrate by use of an interfacing small solder ball. A thin copper pad, coated with a nickel--gold layer, is useful for coupling a chip to a substrate by use of a wirebond interface. The electrical structure includes an electrical coupling of two pads having different thickness, such that the pads are located either on the same surface of a substrate or on opposite sides of a substrate.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a structure, and associated method offormation, in which conductive bonding pads and associated circuitelements of varying height are located on the same substrate.

2. Related Art

A substrate, such as a chip carrier, typically has a top surface and abottom surface wherein either surface, or both surfaces, has conductivebonding pads for electrically coupling the substrate to such devices aselectronic assemblies (e.g., chips) and electronic carriers (e.g,circuit cards). A conductive bonding pad typically contains copper, butmay alternatively contain, inter alia, nickel. Currently, all pads on agiven substrate have the same thickness. A reduction in pad thicknessgenerally conserves space on the substrate as a consequence of theoutward sloping of pad sidewalls from the top of the pad to the bottomof the pad. The outward sloping is generated by the subtractive etchingprocess used to form the pads. The outward, or trapezoidal, slopingcauses the cross-sectional area of the pad at a pad-substrate interfaceto decrease with decreasing pad thickness for a given angular slope. Thereduction of pad cross-sectional area at the pad-substrate interfaceallows the pad centers to be more closely spaced, resulting in anoverall reduction of the substrate surface area required forimplementing the design features of intended applications. The foregoingremarks regarding the use of thin pads to conserve space also apply tocircuit lines coupled to the pads inasmuch as the circuit lines maylikewise be formed by subtractive etching and consequently have slopingsidewalls. Indeed, a pad may be viewed as volumetric section of acircuit line to which a conductive interconnect, such as a wirebondinterconnect or a solder ball, may be electrically and mechanicallycoupled. Thus, both thin pads and associated thin circuit lines improvespace utilization. Pads (and associated circuitizations) may becategorized as to thickness. Such categories include thin pads, thickpads, and medium pads.

A thick pad (and associated circuitization), which typically has athickness between about 17 microns and about 50 microns, can generallybe used for coupling electrical devices and is especially useful forcoupling a large solder ball, such as a solder ball of a ball grid array(BGA), to a substrate for subsequent attachment of the large solder ballto a circuit card.

A thin pad (and associated circuitization), which typically has athickness between about 3 microns and about 10 microns, can be used forcoupling an electronic assembly (e.g., a chip) to a substrate, by use ofa wirebond interface (e.g., a gold wire). However, pads are typicallymade of copper and copper is unsuitable for making a direct attachmentof a chip to a substrate by use of a gold wire. To mitigate thisproblem, the copper pad may be coated with a layer of nickel--gold,wherein a coating of nickel is formed on a top surface of the copperpad, and wherein a coating of gold is formed on the coating of nickel.With the nickel--gold layer over a copper pad, the chip may bewirebonded directly to the gold coating and this wirebond connection isgenerally reliable. A thin or thick copper pad, with an overlyingnickel--gold layer, could also be used for attachment of a BGA solderball. Note that a thin pad without an overlying nickel-gold layergenerally cannot be used for direct attachment of a BGA solder ball,because the soldering process alloys some of the pad metal (e.g.,copper) into the bulk of the solder material (e.g., lead/tin). Thus, ifthe pad is too thin, nearly all of the pad metal may alloy with thesolder material, resulting in an unreliable mechanical and electricalconnection.

A medium pad (and associated circuitization) has a thickness betweenabout 10 microns and about 17 microns. A medium pad is particularlyuseful in flip-chip bonding of a chip to a substrate by use of a smallsolder ball. Such flip-chip bonding may be accomplished by thecontrolled collapse chip connection (C4) technique. The diameter of thesmall solder ball may be nearly an order of magnitude smaller than thediameter of a BGA solder ball (e.g., 2 to 3 mils for a small solder ballversus 25 to 30 mils for a BGA solder ball). The relatively smallersolder ball diameter allows the pad thickness for small solder ballattachment to be less than the pad thickness for BGA solder ballattachment, due to consideration of the alloying of pad metal with thesolder material as discussed supra.

It is to be noted that a BGA solder ball can be directly soldered tonickel--gold coating over a thin copper pad, which conserves space.There is controversy, however, as to whether the solder-gold interfaceis susceptible to joint degradation. Thus, some designers and/or usersmay prefer to couple a BGA solder ball to a substrate by using a thick,uncoated copper pad than by using a nickel--gold coated thin copper pad.The decision of whether to couple a BGA solder ball to a substrate byusing a thick copper pad or a thin nickel--gold coated copper pad istherefore discretionary and involves balancing the space-saving featuresof thin pads against reliability concerns associated with thinnickel--gold coated thin copper pads.

For applications requiring low-power input to a chip and low processingspeed, it may be desirable to have thin circuitization throughout thesubstrate except where thick BGA pads are required. For applicationsrequiring high-power input to a chip and high processing speed, it maybe desirable to have thick circuitization throughout the substrateexcept where thin wirebond pads are required.

Currently, pads and associated circuit lines on a given substrate are ofuniform thickness throughout the substrate. It would be desirable tohave pads and associated circuit lines of differing thicknesses on thesame substrate in order to benefit from the advantages associated witheach pad thickness and circuit line thickness.

SUMMARY OF THE INVENTION

The present invention provides an electronic structure, comprising:

a substrate;

a first circuit line including a first conductive pad and having a firstthickness, wherein the first circuit line is coupled to the substrate;and

a second circuit line including a second conductive pad and having asecond thickness that is unequal to the first thickness, wherein thesecond circuit line is coupled to the substrate, and wherein the secondcircuit line is electrically coupled to the first circuit line.

The present invention also provides a method for forming an electronicstructure, comprising:

providing a substrate;

forming a first circuit line that includes a first conductive pad andhas a first thickness;

coupling the first circuit line to the substrate;

forming a second circuit line that includes a second conductive pad andhas a second thickness that is unequal to the first thickness;

coupling the second circuit line to the substrate; and

electrically coupling the second circuit line to the first circuit line.

The present invention has the advantage of allowing pads and associatedcircuit lines on the same substrate to have different thicknesses, whichenables the benefits associated with each circuit line thickness andeach pad thickness to be realized.

The present invention has the advantage of allowing thick BGA pads andthin wirebond pads to exist on the same substrate.

The present invention has the advantage of allowing thick BGA pads andmedium C4 solder-ball pads to exist on the same substrate.

The present invention has the advantage of allowing thin wirebond padsand medium C4 solder-ball pads to exist on the same substrate.

The present invention has the advantage of allowing applicationsrequiring low-power input to a chip and low processing speed to havethin circuitization throughout the substrate except where thick BGA padsare required.

The present invention has the advantage of applications requiringhigh-power input to a chip and high processing speed to have thickcircuitization throughout the substrate except where thin wirebond padsare required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a front cross-sectional view of a substrate with a platedthrough hole (PTH) and added metal foil layers, in accordance with aninitial step of a preferred embodiment of the process of the presentinvention.

FIG. 2 depicts FIG. 1 with indicated regions to be circuitized tothicknesses of the metal foil layers.

FIG. 3 depicts FIG. 2 after the indicated regions have been circuitizedto form first, second, and third circuit lines.

FIG. 4 depicts a top perspective view of the configuration of FIG. 3.

FIG. 5 depicts FIG. 3 after metallic coatings have been formed on asurface of the first circuit line.

FIG. 6 depicts FIG. 5 after metal has been plated on the metal foil toform metal layers.

FIG. 7 depicts FIG. 6 with indicated regions to be circuitized tothicknesses of the metal layers.

FIG. 8 depicts FIG. 7 after the indicated regions have been circuitizedto form fourth, fifth, and sixth circuit lines.

FIG. 9 depicts a top view of a first preferred embodiment of thestructure of the present invention.

FIG. 10 depicts a front cross-sectional view of a second preferredembodiment of the structure of the present invention.

FIG. 11 depicts a front cross-sectional view of a third preferredembodiment of the structure of the present invention.

FIG. 12 depicts a front cross-sectional view of a fourth preferredembodiment of the structure of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-8 illustrate a preferred embodiment of the method of the presentinvention. FIG. 1 illustrates a front cross-sectional view of asubstrate 10 with a plated through hole (PTH) 12, a top layer of metalfoil 14 on the top surface 18 of the substrate 10, and a bottom layer ofmetal foil 16 on the bottom surface 19 of the substrate 10. Thesubstrate 10 may represent a device such as a chip carrier. The PTH 12has a plated metal inner wall 13 for providing conductive couplingbetween circuitizations to be subsequently formed on both the topsurface 18 and the bottom surface 19. The PTH may be filled with aninsulative material to prevent seepage of matter into the PTH duringsubsequent fabrication steps. The top layer of metal foil 14 on the topsurface 18, having a thickness t₁, may be formed by any known method. Itis common to first form a metal foil of standard thickness exceeding t₁on the top surface 18, followed by chemically etching the metal foildown to the thickness t₁. The thickness t₂ of the bottom layer of metalfoil 16 may be formed by any known method, including a method similar tothat used for forming the thickness t₁ of the top layer of metal foil14. Note that t₂ may be unequal to t₁. The material of the top layermetal foil 14, and of the bottom layer of metal foil 16, may be anymaterial that could be used for forming conductive pads and associatedcircuit lines. A conductive pad typically contains copper, but mayalternatively contain, inter alia, nickel.

FIG. 2 illustrates FIG. 1 after identification of regions to besubsequently circuitized, namely regions 20 and 22 within the top layerof metal foil 14, and region 24 within the bottom layer of metal foil16. FIG. 3 illustrates FIG. 2 after formation of a first circuit line 30of thickness t₁, a second circuit line 32 of thickness t₁, and a thirdcircuit line 34 of thickness t₂, from regions 20, 22, and 24 (see FIG. 2for regions 20, 22, and 24), respectively. The first circuit line 30,second circuit line 32, and third circuit line 34 in FIG. 3 may beformed by any method known in the art, such as by photolithography withsubtractive etching. Employing photolithography includes applying,exposing, developing, etching, and stripping steps. In the applyingstep, photoresist is applied to the open surfaces of the metal foillayers 14 and 16 in FIG. 2. An open surface is defined as a surface thatis open to (i.e., in contact with) the atmosphere. In the exposing step,the photoresist-covered surfaces under which circuitizations will besubsequently formed are selectively exposed to light of a suitablewavelength (e.g., ultraviolet light). With particular reference to FIG.2, the light is selectively directed to surfaces under which the firstcircuit line 30, the second circuit line 32, and the third circuit line34 will be subsequently formed; i.e., to the surface 35 of region 20,the surface 37 of region 22, and the surface 38 of region 24. The lightis also directed to surfaces under which additional circuitizations willbe subsequently formed as will be described infra, namely the opensurfaces 44 of the top layer of metal foil 14 and the open surfaces 46of the bottom layer of metal foil 14, as shown in FIG. 2 and FIG. 3. Thephotoresist that is exposed to the selectively directed light isprotected in the subsequent developing step. In the developing step, thephotoresist is developed away from surfaces not previously exposed (saidsurfaces not shown in FIG. 3). In the etching step, the unprotectedmetal (i.e., unexposed metal) of the metal foil layers 14 and 16 isremoved by chemical etching, resulting in the formation of circuit lines30, 32, and 34 shown in FIG. 3. The removal of the unprotected metalgenerates void space adjacent to circuit lines 30, 32, and 34. This voidspace is not depicted in FIG. 3, because the cross-sectional view ofFIG. 3 does not traverse the void space. The projected widths w₁ and w₂of the first circuit line 30 and the second circuit line 32,respectively, serve to correlate the top view of FIG. 4 with thecross-sectional view of FIG. 3. After the etching step, some metal foil14 and some metal foil 16 remains, namely the metal foil 14 having opensurfaces 44, and the metal foil 16 having open surfaces 46. In thestripping step, the exposed photoresist is stripped away.

FIG. 4 illustrates of top view of the configuration of FIG. 3, showingthe top layer of metal foil 14 and not showing the bottom layer of metalfoil 16. The aforementioned subtractive etching process (described suprain conjunction with FIG. 3) generates a first void space 31 surroundingthe first circuit line 30, and a second void space 33 surrounding thesecond circuit line 32, as shown in FIG. 4. The first void space 31 andthe second void space 33 define the geometric features of the firstcircuit line 30 and the second circuit line 32, respectively. Theprojected widths w₁ and w₂ of the first circuit line 30 and the secondcircuit line 32, respectively, serve to correlate the top view of FIG. 4with the front view of FIG. 3. Although FIG. 4 does not show the thirdcircuit line 34 of FIG. 3, it should be noted that there is void spacearound the third circuit line 34 that defines the geometric features ofthird circuit line 34. While the first circuit line 30 is only onecircuit line within the first void space 31, as illustrated in FIG. 4,the process of the present invention could generate a plurality ofcircuit lines within the first void space 31 such that void space existsbetween each pair of adjacent circuit lines. Similarly, the second voidspace 33 could include a plurality of circuit lines. It should be notedthat a circuit line, such as the first circuit line 30 or the secondcircuit line 32, may include a designated volumetric section (i.e., a"pad") for subsequent coupling with an electrical connector, such as awirebond connector or a solder ball. A "pad" is defined as a volumetricsection of a circuit line to which a conductive interconnect, such as awirebond interconnect or a solder ball, may be electrically andmechanically coupled.

FIG. 5 illustrates FIG. 3 after a metallic coating 40 is formed by anyknown method, such as plating (e.g., electroplating), on a portion 36 ofthe open surface 35 of the first circuit line 30. The metallic coating40 may serve to conductively couple a wirebond interface, such as a goldwire, to the portion 36. The metallic coating 40 may be formed by anymethod known by one skilled in the art. A known method involvesphotolithographic steps comprising applying, exposing, developing,plating, and stripping steps. In the applying step, photoresist isapplied to all currently open surfaces 44 of the first metal foil layer14, the open surfaces 46 of the second metal foil layer 16, the opensurface 35 of the first circuit line 30, the open surface 37 of thesecond circuit line 32, and the open surface 38 of the third circuitline 34. The purpose of applying photoresist to all open surfaces is toprotect all open surfaces from being plated in the subsequent platingstep, except those open surfaces which are exposed in the subsequentexposing step that precedes the plating step. Next, in the exposingstep, light of a suitable wavelength (e.g., ultraviolet light) isselectively directed to portions of the photoresist-covered surfaceswhich will not be subsequently plated by the metallic coating 40. Inparticular, light of the wavelength will not be directed to the portion36 of the open surface 35 of the first circuit line 30. The photoresistthat is exposed to the selectively directed light is protected in thesubsequent developing step. In the developing step, the photoresist isdeveloped away from surfaces not previously exposed to light, namely theportion 3 (i.e., the copper in the first circuit line 30). In theplating step, the metallic coating 40 is plated on the portion 36. Inthe stripping step, the exposed photoresist is stripped away. For someapplications, the metallic coating 40 includes a first metallic coating41 plated on the portion 36, and a second metallic coating 42 plated onthe first metallic coating 41. For example, a wirebond interface of agold wire cannot directly bond with the first circuit line 30 made ofcopper. To solve this particular problem, the metallic coating 40includes a first metallic coating 41 made of nickel, and second metalliccoating 42 made of gold. The second metallic coating 42 couldalternatively be made of, inter alia, palladium. The nickel in the firstmetallic coating 41 acts as a diffusion barrier to prevent gold fromdiffusing into the copper material located underneath the portion 36.The first metallic coating 41 should be at least about 2.5 microns thickin order to effectively serve as a diffusion barrier and also toreliably maintain its structural integrity. The second metallic coating42 should be at least about 0.5 microns thick in order to be reliablybond with a wirebond interface.

FIG. 6 depicts FIG. 5 after the layer of metal foil 14 (see FIG. 5) istransformed into a top metal layer 50 of thickness t₃ that exceeds t₁,and after the layer of metal foil 16 (see FIG. 5) is transformed into atop metal layer 54 of thickness t₄ that exceeds t₂. Returning to FIG. 5,the aforementioned transformations are accomplished in several steps.First, all open surfaces (44, 35, 40, 37, 46, and 38) are covered withphotoresist. Second, all photoresist-covered surfaces, except surfaces44 and 46, are protectively exposed to light of a suitable wavelengthsuch as ultraviolet light. Third, the unexposed photoresist on surfaces44 and 46 is developed away. Fourth, the same metal as is in the toplayer of metal foil 14 is plated on the open surfaces 44 to form,together with underneath top layer metal foil 14, the top metal layer 50shown in FIG. 6. Similarly, the same metal as is in the bottom layer ofmetal foil 16 is plated on the open surfaces 46 to form, together withunderneath bottom layer metal foil 16, the bottom metal layer 54. Theremaining exposed surfaces are protected from being plated.

FIG. 7 illustrates FIG. 6 after identification of regions to besubsequently circuitized, namely region 56 within the top metal layer50, and regions 58 and 60 within the bottom metal layer 54. FIG. 8illustrates FIG. 7 after formation of a fourth circuit line 70 ofthickness t₃, a fifth circuit line 72 of thickness t₄, and a sixthcircuit line 74 of thickness t₄, from regions 56, 58, and 60 (see FIG. 7for regions 56, 58, and 60), respectively. The fourth circuit line 70,fifth circuit line 72, and sixth circuit line 74 in FIG. 8 may be formedby any method known in the art, such as by subtractive etching. Withsubtractive etching in consideration of the existing exposed photoresist(discussed supra in connection with FIG. 6), the unprotected metal(i.e., unexposed metal) of the top metal layer 50, as well as theunprotected metal of the bottom metal layer 54, is removed by chemicaletching so as to form the fourth circuit line 70, the fifth circuit line72, and the sixth circuit line 74. Next, the exposed photoresist isstripped away. It should be noted that any volumetric portion of circuitlines 70, 72, and 74 may constitute a "pad" for subsequent coupling withan electrical connector, such as a wirebond interconnect or a solderball.

The preceding steps, resulting in the electronic structure illustratedin FIG. 8, for forming the top metal layer 50 and the bottom metal layer54 and subsequently forming circuit lines 70, 72, and 74, may berepeated to form additional circuitization layers. In particular, therelevant steps (applying photoresist, selectively exposing thephotoresist, developing away unexposed photoresist, plating metal onunexposed surfaces, and subtractive etching to define circuit linegeometric features) may be used to form a top circuitization layer ofthickness t₃, (exceeding t₃) on the top surface 18 of the substrate 10,and a circuitization layer of thickness t₄, (exceeding t₄) on the bottomsurface 19 of the substrate 10. In this manner an arbitrary finitenumber of circuitization layers may be generated on a substrate by themethod of the present invention. Each formed circuitization layer has agreater thickness than the prior formed circuitization layers on thesame surface (top surface 18 or bottom surface 19) of the substrate 10.

After all circuitization layers have been formed, a portion of anycircuitization layer may be covered by a protective coating. Suchcoatings may include, inter alia, an organic photoresist, a polyimide,an acrylic, or an epoxy. As an example, the protective coating 78 inFIG. 8 covers a portion of the fifth circuit line 72 and the secondcircuit line 34.

Any two circuit lines of different thickness may be formed to beconductively coupled such that a pad on one of the two circuit linescouples the substrate to an electronic assembly, such as a chip, and theother of the two circuit lines couples the substrate to an electroniccarrier, such as a circuit card. See, e.g, FIGS. 10-12, to be discussedinfra, for various illustrative electrical structures of the presentinvention. FIG. 8 illustrates that first circuit line 30 may beconductively coupled with fourth circuit line 70, and third circuit line34 may be conductively coupled with fifth circuit line 72, whichillustrate the electrical coupling of two circuit lines of differentthickness located on the same surface of a substrate. FIG. 8 alsoillustrates that first circuit line 30 may be mechanically coupled withfourth circuit line 70, and third circuit line 34 may be mechanicallycoupled with fifth circuit line 72, wherein the mechanical coupling maybe effectuated by any method known to one of ordinary skill in the artsuch as by abutting or adhesive contact. The second circuit line 32 maybe conductively coupled with sixth circuit line 74 by use of the PTH 12,thereby electrically coupling two circuit lines of different thicknesslocated on opposite surfaces of a substrate. Any known variation of theelectrical structure illustrated by the PTH 12 may be used toelectrically couple two circuit lines of different thickness located onopposite surfaces of a substrate. For example, the second circuit line32 may be electrically coupled to a first PTH, the sixth circuit line 74may be electrically coupled to a second PTH, and the first PTH may beelectrically coupled to the second PTH by a conductive plane within thesubstrate or by a plurality of electrically coupled conductive planeswithin the substrate.

The thicknesses t₁, t₂, t₃, and t₄ of the circuit lines (and associatedpads) in FIGS. 1-8 should be in the range of about 3 microns to about 50microns, as discussed in the "Related Art" section.

While FIGS. 1-8 illustrate thickness-varying circuit lines (andassociated pads) on both the top surface 18 and the bottom surface 19 ofthe substrate 10, the present invention includes embodiments havingthickness-varying circuit lines on either the top surface 18 or thebottom surface 19, but not on both surfaces.

FIGS. 9-12 illustrate preferred electronic structures that could beformed by the method described supra and illustrated in FIGS. 1-8. FIG.9 illustrates a top view of a first electrical structure 80, inaccordance with a first preferred structural embodiment of the presentinvention. The first electrical structure 80 includes a substrate 90which may represent a device such as a chip carrier. As stated in the"Related Art" section, a fine circuitization (including pads) has athickness between about 3 microns and about 10 microns, a mediumcircuitization (including pads) has a thickness between about 10 micronsand about 17 microns, and a thick circuitization (including pads) has athickness between about 17 microns and about 50 microns. In FIG. 9,circuit line 92 has a fine circuitization, circuit lines 94 and 100 eachhave a medium circuitization, and circuit lines 96 and 104 each have athick circuitization. Note that the relative thicknesses of circuitlines 92, 94, 96, 100, and 104 are not explicitly shown because FIG. 9is a top view. FIG. 9 shows the thin circuit line 92 to be coupled thesubstrate 90, wherein the thin circuit line 92 is electrically andmechanically coupled to a medium circuit line 94, and wherein the mediumcircuit line 94 is electrically and mechanically coupled to a thickcircuit line 96. The preceding mechanical coupling between circuit lines92 and 94, and between circuit lines 94 and 96, may be effectuated byany method known to one of ordinary skill in the art such as byabutting, adhesive contact. A thin pad 93, which is suitable forcoupling with a wirebond interconnect such as a gold wire, is positionedat an end of the thin circuit line 92. The wirebond interconnect may beused to electrically couple the thin pad 93 to an electronic assemblysuch as a chip. A thick pad 98, which is suitable for coupling with alarge solder ball such as a BGA solder ball, is positioned at an end ofthe thick circuit line 96. The large solder ball may be used toelectrically couple the thick pad 98 to an electronic carrier such as acircuit card. FIG. 9 also shows a medium circuit line 100 coupled to thesubstrate 90, wherein the medium circuit line 100 is coupled to a thickcircuit line 104. A medium pad 102, which is suitable for coupling witha small solder ball, is positioned within the medium circuit line 100.The small solder ball may be used to electrically couple the medium pad102 to an electronic assembly, such as a chip, by any suitable methodsuch as controlled collapse chip connection (C4). A thick pad 106, whichis suitable for coupling with a large solder ball such as a BGA solderball, is positioned within the thick circuit line 104.

FIG. 10 illustrates a front cross-sectional view of a second electricalstructure 200, in accordance with a second preferred structuralembodiment of the present invention. The second electrical structure 200includes a substrate 204 which may represent a device such as a chipcarrier. In FIG. 10, an electronic assembly 240 (e.g., a chip) within ancavity 207 in a substrate 204 is coupled to the substrate 204 by use ofan adhesive interface 242. A first circuit line 210 is mechanicallycoupled to a bottom surface 206 of the substrate 204, and isconductively coupled to the electronic assembly 240 by use of a wirebondinterconnect 244, wherein the mechanical coupling may be effectuated byany method known to one of ordinary skill in the art such as byabutting, adhesive contact, or hot press lamination. The wirebondinterconnect 244 couples the electronic assembly 240 to an open surface216 of a metallic coating 211. The metallic coating 211 is on a portion217 of the bottom surface 218 of the first circuit line 210. Themetallic coating 211 includes a first metal coating 212 on the bottomsurface 218, and a second metal coating 214 on the first metal coating212. The first circuit line 210 has a thickness t₅ which may be anythickness in the range of about 3 microns to about 50 microns,preferably in a range of about 3 microns to about 10 microns. As anexample, the first circuit line 210 may include copper, the first metalcoating 212 may include nickel, the second metal coating 214 may includegold, and the wirebond interconnect 244 may include a gold wire. The"pad" to which the wirebond interconnect 244 is attached includes thevolumetric portion 209 of the first circuit line 210 that is beneath themetallic coating 211. A second circuit line 220 is mechanically coupledto the bottom surface 206 of the substrate 204, and is conductivelycoupled to the first circuit line 210, wherein the mechanical couplingmay effectuated by any method known to one of ordinary skill in the artsuch as by abutting, adhesive contact, or hot press lamination. Thesecond circuit line 220 has a thickness t₆ which may be any thickness ina range of about 3 microns to about 50 microns, other than t₅. While t₆is shown in FIG. 10 as exceeding t₅, t₆ may nevertheless be less thant₅. A third circuit line 230, of thickness t₇ where t₇ ≠ t₆ and t₇ >t₅,is mechanically coupled to the bottom surface 206 of the substrate 204and is conductively coupled to the second circuit line 220, wherein themechanical coupling may be effectuated by any method known to one ofordinary skill in the art such as by abutting, adhesive contact, or hotpress lamination. The third circuit line 230 includes a pad 232 which iscoupled to a solder ball 250, wherein the pad 232 includes thevolumetric portion of the third circuit line 230 that interfaces withthe solder ball 250. If the solder ball 250 is a BGA solder ballconnected to an electronic device 260 such as an electronic carrier(e.g., circuit card), where the BGA solder ball has a diameter in arange of about 25 mils to about 30 mils, then t₇ should be in the rangeof about 17 microns to about 50 microns. If the solder ball 250 is asmall solder ball connected to an electronic device 260 such as anelectronic assembly (e.g., chip), where the small solder ball has adiameter of about an order of magnitude less than the diameter of a BGAsolder ball (i.e, about 2 to about 3 mils), then t₇ should be in a rangeof about 10 microns to about 50 microns, preferably in a range of about10 microns to about 17 microns.

FIG. 11 illustrates a front cross-sectional view of a third electricalstructure 300, in accordance with a third preferred structuralembodiment of the present invention. The third electrical structure 300includes a substrate 304 which may represent a device such as a chipcarrier. In FIG. 11, an electronic assembly 340 (e.g., a chip) on a topsurface 305 of a substrate 304 is coupled to the substrate 304 by use ofan adhesive interface 342. A first circuit line 310 is mechanicallycoupled to the top surface 305 of the substrate 304, and is conductivelycoupled to the electronic assembly 340 by use of a wirebond interconnect344, wherein the mechanical coupling may be effectuated by any methodknown to one of ordinary skill in the art such as by abutting, adhesivecontact, or hot press lamination. The wirebond interconnect 344 couplesthe electronic assembly 340 to an open surface 316 of a metallic coating311. The metallic coating 311 is on a portion 317 of the top surface 318of the first circuit line 310. The metallic coating 311 includes a firstmetal coating 312 on the top surface 318 of the first circuit line 310,and a second metal coating 314 on the first metal coating 312. The firstcircuit line 310 has a thickness t₈ which may be any thickness in therange of about 3 microns to about 50 microns, preferably in a range ofabout 3 microns to about 10 microns. The first circuit line 310 and themetallic coating 311 may include the same materials as stated supra inthe example for the first circuit line 210 and metallic coating 211 inFIG. 10. The "pad" to which the wirebond interconnect 344 is attachedincludes the volumetric portion 309 of the first circuit line 310 thatis beneath the metallic coating 311. A second circuit line 320, ofthickness t₉ where t₉ is unequal to t₈ and preferably greater than t₈,is mechanically coupled to the bottom surface 306 of the substrate 304,and is conductively coupled to the first circuit line 310 by a PTH 308,wherein the mechanical coupling may be effectuated by any method knownto one of ordinary skill in the art such as by abutting, adhesivecontact, or hot press lamination. The second circuit line 320 includes apad 332 which is coupled to a solder ball 350, wherein the pad 332includes the volumetric portion of the second circuit line 320 thatinterfaces with the solder ball 350. The solder ball 350 may be coupledto an electronic device 360 such as an electronic carrier (e.g., circuitcard) or an electronic assembly (e.g., chip). Ranges of values for thethickness t₉ and the solder ball 350 diameter are based on the sameconsiderations as are the ranges of values for thickness t₇ and solderball 250 diameter, respectively, as discussed supra for FIG. 10.

FIG. 12 illustrates a front cross-sectional view of a fourth electricalstructure 400, in accordance with a fourth preferred structuralembodiment of the present invention. The fourth electrical structure 400includes a substrate 404 which may represent a device such as a chipcarrier. In FIG. 12, a first circuit line 410 is coupled to a topsurface 405 of a substrate 404. An electronic assembly 440 (e.g., achip) is conductively coupled to the first circuit line 410 by use of aninterfacing small solder ball 442 such as a C4 solder ball having adiameter between about 2 mils and about 3 mils. The first circuit line410 has a thickness t₁₀ which may be any thickness in the range of about10 microns to about 50 microns, preferably in a range of about 10microns to about 17 microns. The "pad" to which the small solder ball442 is attached includes the volumetric portion 409 of the first circuitline 410 that is beneath the small solder ball 442. A second circuitline 420, of thickness t₁₁ where t₁₁ is unequal to t₁₀, is coupled tothe bottom surface 406 of the substrate 404, and is conductively coupledto the first circuit line 410 by a PTH 408. The second circuit line 420includes a pad 432 which is coupled to a solder ball 450, wherein thepad 432 includes the volumetric portion of the second circuit line 420that interfaces with the solder ball 450. The solder ball 450 may becoupled to an electronic device 460 such as an electronic carrier (e.g.,circuit card) or an electronic assembly (e.g., chip). Ranges of valuesfor the thickness t₁₁ and the solder ball 450 diameter are based on thesame considerations as are the ranges of values for thickness t₇ andsolder ball 250 diameter, respectively, as discussed supra for FIG. 10.

While preferred and particular embodiments of the present invention havebeen described herein for purposes of illustration, many modificationsand changes will become apparent to those skilled in the art.Accordingly, the appended claims are intended to encompass all suchmodifications and changes as fall within the true spirit and scope ofthis invention.

We claim:
 1. A method for forming an electronic structure,comprising:providing a substrate; forming a first circuit line thatincludes a first conductive pad and has a first thickness; coupling thefirst circuit line to the substrate; forming a second circuit line thatincludes a second conductive pad and has a second thickness that isunequal to the first thickness; coupling the second circuit line to thesubstrate; and electrically coupling the second circuit line to thefirst circuit line.
 2. The method of claim 1, further comprisingmechanically coupling the second circuit line to the first circuit line.3. A method for forming an electronic structure, comprising:providing asubstrate; forming a first circuit line that includes a first conductivepad and has a first thickness; coupling the first circuit line to thesubstrate; forming a second circuit line that includes a secondconductive pad and has a second thickness that is unequal to the firstthickness; coupling the second circuit line to the substrate; andelectrically coupling the second circuit line to the first circuit line,including:forming a third circuit line, coupled to the substrate, havinga third thickness that is unequal to both the first thickness and thesecond thickness; electrically coupling a portion of the third circuitline with a portion of the first circuit line; and electrically couplinga portion of the third circuit line with a portion of the second circuitline.
 4. The method of claim 1, wherein the step of forming a firstcircuit line comprises forming the first conductive pad at an end of thefirst circuit line, and wherein the step of forming a second circuitline comprises forming the second conductive pad at an end of the secondcircuit line.
 5. The method of claim 1, further comprising:selecting acircuit line that includes the first circuit line and the second circuitline; forming a protective coating over a portion of the circuit line.6. The method of claim 1, wherein the step of coupling the first circuitline to the substrate includes mechanically coupling the first circuitline to a first surface of the substrate, and wherein step of couplingthe second circuit line to the substrate includes mechanically couplingthe second circuit line to a second surface of the substrate.
 7. Themethod of claim 6, wherein the electrically coupling stepcomprises:forming a plated through hole (PTH) through a thickness of thesubstrate; coupling a portion of the first circuit line to a first endof the PTH; and coupling a portion of the second circuit line to asecond end of the PTH.
 8. The method of claim 1, wherein the step ofcoupling the first circuit line to the substrate includes mechanicallycoupling the first circuit line to a surface of the substrate, andwherein the step of coupling the second circuit line to the substrateincludes mechanically coupling the second circuit line to the surface.9. A method for forming an electronic structure, comprising:providing asubstrate; forming a first circuit line that includes a first conductivepad and has a first thickness; coupling the first circuit line to thesubstrate; forming a second circuit line that includes a secondconductive pad and has a second thickness that is unequal to the firstthickness; coupling the second circuit line to the substrate;electrically coupling the second circuit line to the first circuit line;coupling a first solder ball to the first conductive pad; coupling anelectronic assembly to the first solder ball; coupling a second solderball to the second conductive pad; and coupling an electronic carrier tothe second solder ball.
 10. The method of claim 9, wherein the step ofcoupling a second solder ball to the second conductive pad includescoupling the second solder ball, having a diameter that is unequal to adiameter of the first solder ball, to the second conductive pad.
 11. Themethod of claim 1, wherein the first conductive pad includes a metalliclayer, further comprising:forming a first metallic coating over themetallic layer; and forming a second metallic coating over the firstmetallic coating, said first metallic coating inhibiting diffusion of ametal from the second metallic coating into the metallic layer.
 12. Themethod of claim 11, further comprising:forming a wirebond interconnectto the first conductive pad at the second metallic coating; coupling anelectronic assembly to the wirebond interconnect; forming a solder ballcoupled to the second conductive pad; and coupling an electronic carrierto the solder ball.
 13. A method for forming an electronic structure,comprising:providing a substrate; forming a first circuit line thatincludes a first conductive pad and has a first thickness, wherein thefirst conductive pad includes a metallic layer, and wherein the metalliclayer includes copper; coupling the first circuit line to the substrate;forming a second circuit line that includes a second conductive pad andhas a second thickness that is unequal to the first thickness; couplingthe second circuit line to the substrate; electrically coupling thesecond circuit line to the first circuit line; forming a first metalliccoating over the metallic layer, wherein the first metallic coatingincludes nickel; forming a second metallic coating over the firstmetallic coating, wherein the first metallic coating inhibitingdiffusion of a metal from the second metallic coating into the metalliclayer, and wherein the metal of the second metallic coating includesgold or palladium; forming a wirebond interconnect to the firstconductive pad at the second metallic coating, wherein the wirebondinterconnect includes gold; coupling an electronic assembly to thewirebond interconnect; forming a solder ball coupled to the secondconductive pad; and coupling an electronic carrier to the solder ball.